Heat-shrinkable films have been extensively used, e.g., for labeling bottles, batteries or electrolytic condensers, and for wrapping containers and other products. Such heat-shrinkable films are formed of polyvinyl chloride, polystyrene, polypropylene or polyester, and they are required to have good sealing and shrinking properties and good driving and crack-resistance properties along the longitudinal direction as well as good heat resistance, chemical resistance, weatherability and printability.
However, conventional heat-shrinkable polyester films for labeling or wrapping which are prepared by primarily drawing undrawn sheets in the transverse direction tend to undergo shrinkage in the drawn transverse direction, thereby exhibiting poor mechanical properties, i.e., low tensile strength and reduced driving properties (frequent film cuts), in the longitudinal direction, especially during withdrawing or post-processing.
In addition, conventional heat-shrinkable polyester films have the problem that cracks are often generated after thermally shrunk for labeling or wrapping due to their high degree of crystallization.
A typical container wrapping or labeling process using a film drawn in the transverse direction is shown in FIG. 1. Referring to FIG. 1, a predetermined pattern is printed on a film, the printed film is slit into a specified width, and a sleeve seaming process is conducted to obtain a sleeve form, which is wound to prepare a sleeve roll. The sleeve sheet is cut to a predetermined length and the cut sleeve is placed around a target, and subjected to heat-shrinking in a shrinking tunnel. Such a conventional process, however, has various problems in that it is very complicated and it also suffers from low productivity due to discontinuity between before and after the sleeve roll preparation process.
In order to overcome the above-mentioned problems, Japanese Laid-open Patent Publication No. 1998-77335 discloses a method for preparing a film by copolymerizing using a naphthalenedicarboxylic acid component; and Japanese Laid-open Patent Publication Nos. 1988-139725, 1994-122152, 1995-53737, 1995-216107 and 1995-216109 disclose that the shrinkage rate of a film can be controlled by blending a polyethylene terephthalate or polybuthylene terephthalate in a particular ratio, or by copolymerizing a dicarboxylic acid component such as terephthalic acid and isophthalic acid with a diol component such as ethylene glycol and 1,4-cyclohexanedimethanol. However, these methods still do not provide satisfactory improvements.